In the Mulller-Rochow direct synthesis of methylchiorosilanes, methychloride is reacted with silicon in the presence of a copper catalyst and suitable promoters to give methylchlorosilanes. In addition to the highest possible productivity based on the amount of silanes formed per time unit and reaction volume, the highest possible selectivity, based on the target product dimethyldichlorosilane, is also required. Dimethyldichlorosilane is required, for example, for the preparation of linear polysiloxanes.
The direct synthesis can be carried out batchwise or continuously, with the continuous version being preferred in industrial production. The continuous direct synthesis is carried out in fluidized-bed reactors in which methyl chloride is employed simultaneously as fluidization medium and reactant. The silicon required is firstly ground to a powder having a particle size of at most 700 .mu.m, and mixed with copper catalyst and promoters to provide the catalyst composition.
In continuous operation of a reactor, the production rate, based on methylchlorosilanes, and the selectivity, based on the target product dimethyldichlorosilane, drop after a substantially stable production phase. For this reason, the production campaign must be terminated after a certain time. A production campaign therefore usually lasts from only a few days to several weeks.
After termination of a production campaign, the reactor is emptied, refilled with silicon, copper catalyst and promoters, and restored to the reaction conditions. After a certain induction phase, the formation of crude silane begins. This is followed by the start phase, i.e. the reaction initially proceeds with low selectivity and reactivity. The reactor then subsequently reaches the stable production phase again. It can be seen from this that the economic efficiency of the direct synthesis can be increased with constant selectivity by increasing the production rate.
The reactivity and selectivity in the direct synthesis are highly dependent on the catalysts and promoters employed. U.S. RE 33,452 describes, for example, a direct synthesis using a catalyst combination of the elements or compounds of copper, zinc and tin. The ratio of copper, zinc and tin to one another has a considerable effect on the process, in particular on the productivity and selectivity, while the form in which the catalysts are introduced into the catalyst composition, for example as metal, alloy or chemical compound, is of secondary importance.
In "Catalyzed Direct Reactions of Silicon", edited by K. M. Lewis and D. G. Rethwisch, Elsevier, 1993, Chapter 1 "Commercial Production of Silanes by the Direct Synthesis", B. Kanner, K. M. Lewis, page 12, various copper catalysts are described which are used in the process carried out on a large industrial scale. For example, it is shown that copper(I) chloride can be used as the only copper component, but a mixture of copper oxide and copper(I) chloride is sometimes also used. By use of such mixtures, the induction period is shortened and the reactivity is increased; the effect of these mixtures on the selectivity has not been reported.
In "Handbook of Heterogeneous Catalysis", edited by G. Ertl, H. Knozinger, J. Weitkamp; Volume 4, Wiley-VCH; Chapter 2 (Inorganic Reactions), 2.5 The Direct Process to Methylchlorosilane (Muller-Rochow Synthesis) by B. Pachaly, page 1788, it is stated that the advantage of copper oxide over CuCl in the large-scale industrial process is that the conversion of copper oxide into copper chloride which takes place in the reaction system is slower than the reaction of CuCl to Cu.sub.3 Si, which represents the actual reaction center. The steady-state concentration of CuCl is thus very low, less metallic copper is formed by the redox reaction of copper chloride with metallic silicon, and the undesired side reactions initiated by metallic copper, which reduce the selectivity, occur to a reduced extent. At the same time, the possibility of employing various mixtures of different copper catalysts is disclosed.
In "Chemistry and Technology of Silicones", W. Noll; Academic Press, 1968, page 29, it is stated that copper(I) chloride can be employed instead of metallic copper, but in this case the proportion of methyldichlorosilane increases, i.e. the selectivity drops.
The chemical nature of the copper catalyst employed in the catalyst system has a massive effect on the reactivity and selectivity. Copper(I) chloride gives very good reactivity, with the selectivity dropping greatly with advancing reaction time; copper oxide shows long-lasting good selectivity at the same time as moderate reactivity. If a mixture of copper oxide and copper chloride is employed as catalyst, the reaction behavior is determined by one of the two individual components, depending on the mixing ratio.